The invention relates to a computer tomograph with a charge-integrating read amplifier having a plurality of gain factors for a data acquisition system as well as a control circuit for the gain factors.
U.S. Pat. No. 5,363,055 discloses a programmable read amplifier with a plurality of gain factors for a data acquisition system of a computer tomograph. The read amplifier includes an operational amplifier with an inverting input, a non-inverting input and an output as well as a plurality of capacitors which can be connected parallel to the operational amplifier, between the inverting input and the output of the operational amplifier, by means of switches.
Those skilled in the art will understand that the term xe2x80x9cframexe2x80x9d as used herein in its conventional manner refers to a complete shot of CT data from one exposure. For example, in a CT detection scheme which utilizes 1000 detectors, one frame would comprise one thousand parts of data derived respectively from each of the 1000 detectors for 1 shot period (i.e., exposure). A typical shot period is around a millisecond.
A reset switch first short-circuits all parallel connected capacitors in order to remove residual charges from the capacitors before application of an input signal from a photodiode to the input of the amplifier. Subsequently, the accumulated charge of the photodiode is applied to the input of the amplifier and transferred to the parallel connected capacitors. Finally, individual switches are actuated by a controller in order to connect the capacitors to the reference potential and to transfer their charge to the other parallel capacitors. As a result of this step, the gain factor is controlled so as to reduce the necessary resolution of the subsequent analog-to-digital converter.
It is a drawback of the known read amplifier that all capacitors are connected in parallel during the transfer of the charge of the photodiode. This results in a very high capacitance parallel to the operational amplifier, causing additional noise in the read amplifier. If subsequently individual capacitors are connected to the reference potential, a further noise (kTC noise) occurs in the read amplifier.
Starting from this state of the art, it is an object of the invention to provide a computer tomograph of the kind set forth in which noise additional to the shot noise is avoided and in which, in comparison with the required overall dynamic range of the read amplifier, a smaller local dynamic range is feasible in the individual amplifier stages, without the DQE (Detection Quantum Efficiency) being degraded.
The shot noise of the signal arises from the square root thereof. The local dynamic range in the individual amplifier stages of the read amplifier, therefore, could correspond to the signal-to-noise ratio for the highest signal if the signal of the next frame were known exactly in advance. The invention utilizes the fact that the input signal of a computer tomograph varies only slowly between successive frames and that information as regards the next signal can be extracted therefrom.
This object is specifically achieved in a computer tomograph according to the invention in that the control circuit automatically selects the gain factor in dependence on the expected integrated input signal of the next frame, the expectation being based on the maximum possible relative variation of the integrated input signal between two successive frames.
The choice between two or more gain factors of. the read amplifier is preferably made by comparison of the integrated input signal with reference values as described in claim 2. Reliable operation with full resolution of the signal and the shot noise is ensured when the reference values are determined in conformity with claim 3.
Claim 4 discloses an advantageous circuit arrangement of a read amplifier for a computer tomograph. In the case of an amplifier having only two gain factors, the circuitry for the control of the read amplifier can be formed by means of a threshold switch having two stable states, notably a Schmitt trigger, and an inverter.
When use is made of CMOS technology, the read amplifier can be arranged in the direct vicinity of a photosensor (pixel) of the computer tomograph which supplies the input signal.